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Re^2: Is map's sub block really being evaluated in list context? wantarray() returns undef..

by moritz (Cardinal)
on Oct 20, 2011 at 10:35 UTC ( #932615=note: print w/replies, xml ) Need Help??


in reply to Re: Is map's sub block really being evaluated in list context? wantarray() returns undef..
in thread Is map's sub block really being evaluated in list context? wantarray() returns undef..

Curiously, if you define your own sub which takes a block, wantarray treats it as a normal routine:
$ perl -wE 'sub f(&) { shift->() }; say f { wantarray }' 1

I wonder if this inconsistency is worth a bug report.

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Re^3: Is map's sub block really being evaluated in list context? wantarray() returns undef..
by BrowserUk (Pope) on Oct 20, 2011 at 15:07 UTC

    I suspect that as a built-in, map avails itself of more optimisations than it would be safe to apply to a user-written function.

    I know from experience that if you implement a callback interface in XS using the full belts'n'braces approach described in perlcall, then the result is far less efficient than the carefully optimised callback interface used by List::Util::reduce().

    The downside of trying to match that level of optimisation with your own XS code is that it is very easy to create the situation whereby the callback leaks memory if it contains closures. (The early versions of reduce() suffered from the same problems.)

    I think it would be very difficult, if not impossible to compile user-written, runtime-compiled subs taking callbacks, to be as efficient as map. And to 'fix the inconsistency' would essentially require pessimising the implementation of map

    So no, I don't think it is worth a bug report. At most, a documentation change noting the difference might be useful.


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[choroba]: but undef %hash and %hash = () both work, too, but the first one keeps the memory allocated, while the latter makes it available for other parts of the program.
[choroba]: iirc
[perldigious]: karlgoethebier: Well it is a pretty old and complicated (for me) bit of code I wrote (poorly by my current standards), so I'm expecting everything to break when I add the scoping and find out what else is undesireably scope changed. :-)
[perldigious]: Ah, thanks choroba, that sort of thing was precisely what I was wondering when I asked.
[perldigious]: I didn't want to tie up memory unecessarily basically, I wanted to "delete" it specifically to free it up, and wasn't sure I was even accomplishing that.
[stevieb]: perldigious You should start by writing some unit tests. That'll ensure current functionality doesn't break with changes.

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